Of more than 600 people detained in the United States so far, a quarter of them were initially held on immigration law violations, says Jeanne Butterfield, director of American Immigration Lawyers Association in Washington.

And the power of the Immigration and Naturalization Service (INS) to detain foreigners will only grow under antiterrorism bills approved last week by both the House and the Senate.

For the first time, the INS would be able to hold people for seven days without any charges or evidence that they pose a threat or flight risk, as is presently required. The attorney general or INS commissioner would simply have to certify that the government has reason to believe the detainee is engaging in terrorist activity.

Washington immigration lawyer Michael Maggio says he fears potential abuses, such as agents sweeping up people whose only crime is associating with unpopular causes.

It's perfectly legal to hold someone on an immigration charge if the government's real interest is a criminal investigation - as long as the underlying charge is legitimate, says former INS counsel David Martin.

But, Georgetown law professor David Cole says, such detentions become problematic if investigators use immigration charges to avoid triggering the greater protections given criminal defendants, or if those held are denied access to the safeguards guaranteed by INS regulations.

Unlike criminal defendants, detainees held on visa charges do not have the right to free legal counsel. Once deportation proceedings begin, detainees may retain counsel at their own expense, and the INS is supposed to provide a list of attorneys available to assist them.

But lawyers representing detainees say the INS has denied them access to their clients even after the attorneys were retained.

Albader Al-Hazmi, a University of Texas medical resident was detained for a week beginning Sept. 12 before his attorney, Jerry Goldstein of San Antonio, was able to reach him. In the meantime, federal agents brought Al-Hazmi to New York from San Antonio and released him after 12 days without filing charges. "You ought to have some reason given or recourse" to challenge the detention, Mr. Goldstein says.

In New Jersey, an Egyptian immigrant faces deportation and has been locked up for a month. His mistake, according to his attorney, Hani Khoury of Hackensack, N. J.: overstaying his visa and speaking Arabic when investigators knocked on his door to ask about two of the suspected Sept. 11 hijackers who once lived in the building.

Once deportation proceedings are started, detainees have the right to a bond redetermination hearing, a trial before an immigration judge, and appeals to higher courts.

Charges are supposed to be filed within 24 hours, but Attorney General John Ashcroft has changed the regulations to allow for detention for up to 48 hours or a "reasonable period."

The new legislation would broaden the definition of terrorist supporters to include those who provide material support. This could sweep up people who thought they were simply donating money to a hospital run by the Afghan Northern Alliance, for instance, Cole says.

In addition, the new law grants the attorney general and the INS commissioner power to detain individuals specially certified as terrorist suspects. These detainees would have little recourse to appeal the detention.